 /*******************************************************************************
  * Copyright (c) 2004, 2005 IBM Corporation and others.
  * All rights reserved. This program and the accompanying materials
  * are made available under the terms of the Eclipse Public License v1.0
  * which accompanies this distribution, and is available at
  * http://www.eclipse.org/legal/epl-v10.html
  *
  * Contributors:
  * IBM Corporation - initial API and implementation
  *******************************************************************************/
 package org.eclipse.core.internal.registry;

 /**
  * Monitor ensuring no more than one writer working concurrently.
  * Multiple readers are allowed to perform simultaneously.
  *
  * This class was borrowed from org.eclipse.jdt.internal.core.search.indexing.
  */
 public class ReadWriteMonitor {

     /**
      * <0 : writing (cannot go beyond -1, i.e one concurrent writer)
      * =0 : idle
      * >0 : reading (number of concurrent readers)
      */
     private int status = 0;

     private Thread writeLockowner;

     /**
      * Concurrent reading is allowed
      * Blocking only when already writing.
      */
     public synchronized void enterRead() {
         if (writeLockowner == Thread.currentThread())
             return;
         while (status < 0) {
             try {
                 wait();
             } catch (InterruptedException e) {
                 // ignore
 }
         }
         status++;
     }

     /**
      * Only one writer at a time is allowed to perform
      * Blocking only when already writing or reading.
      */
     public synchronized void enterWrite() {
         if (writeLockowner != Thread.currentThread()) {
             while (status != 0) {
                 try {
                     wait();
                 } catch (InterruptedException e) {
                     // ignore
 }
             }
 // System.out.println(this + "lockowner:" + Thread.currentThread());
 writeLockowner = Thread.currentThread();
         }
         status--;
     }

     /**
      * Only notify waiting writer(s) if last reader
      */
     public synchronized void exitRead() {
         if (writeLockowner == Thread.currentThread())
             return;
         if (--status == 0)
             notifyAll();
     }

     /**
      * When writing is over, all readers and possible
      * writers are granted permission to restart concurrently
      */
     public synchronized void exitWrite() {
         if (writeLockowner != Thread.currentThread())
             throw new IllegalStateException ("Current owner is " + writeLockowner); //$NON-NLS-1$
 if (++status == 0) {
             // System.out.println(this + "exitWrite:" + Thread.currentThread());
 writeLockowner = null;
             notifyAll();
         }
     }

     public String toString() {
         StringBuffer buffer = new StringBuffer ();
         buffer.append(this.hashCode());
         if (status == 0) {
             buffer.append("Monitor idle "); //$NON-NLS-1$
 } else if (status < 0) {
             buffer.append("Monitor writing "); //$NON-NLS-1$
 } else if (status > 0) {
             buffer.append("Monitor reading "); //$NON-NLS-1$
 }
         buffer.append("(status = "); //$NON-NLS-1$
 buffer.append(this.status);
         buffer.append(")"); //$NON-NLS-1$
 return buffer.toString();
     }
 }

